1. Field of the Invention
The invention relates to a slip control system.
2. Description of the Related Art
In the related art, an automatic transmission is structured to have a torque converter acting as a fluid coupling for transmitting the rotation of an engine, as transmitted through a crankshaft, to the input shaft of a speed change unit through the torque converter.
This torque converter is composed of a pump impeller, a turbine runner, a stator, a lockup clutch unit and a damper unit. The rotation of the engine is transmitted through a front cover to the pump impeller, and a turbine runner is rotated by the flow of oil, as established according to the rotation of the pump impeller, so that its rotation is transmitted to the input shaft of the speed change unit.
The lockup clutch unit is equipped with a clutch plate that is arranged to move axially, and a friction member is attached to the clutch plate. When a predetermined vehicle speed is achieved after a start of the vehicle, the friction member, as attached to the clutch plate, and the front cover come into contact to apply the lockup clutch unit. As a result, the rotation of the engine is transmitted to the input shaft directly rather than through the torque converter, so that the mileage can be accordingly improved.
In order to improve the mileage, it is conceivable to apply the lockup clutch unit in a lower vehicle speed region than the predetermined vehicle speed region. When the engine is run, there is a possibility that a torque fluctuation will arise in the combustion chamber of the engine. If this torque fluctuation occurs when the vehicle is run at a low speed, the vehicle cannot be run stably.
Thus, there has been provided (as disclosed in Japanese Patent Publication No. 13060/1982) an automatic transmission which allows the vehicle to be run stably under these conditions by sliding the lockup clutch unit to perform a slip control. Therefore, if the torque fluctuation occurs in the engine in a low vehicle speed range, the fluctuation can be absorbed by the lockup clutch unit. During the slip control, the friction member, as attached to the clutch plate, and the front cover slide relative to each other generating a frictional heat and deteriorating the friction member. Thus, the region for the slip control, i.e., the slip control region is set to enhance the durability of the friction member. However, in the automatic transmission of the related art, the slip control cannot be properly performed as a result of production dispersion and aging of the lockup clutch unit, and the amount of frictional heat, i.e., the calorific value may not be maintained within a proper range. Therefore, the lifetime of the lockup clutch unit is shortened or the mileage obtained by the vehicle is deteriorated.